In combustors gaseous, liquid or solid fuels and air are burnt to generate heat. The gas resulting from burning fuel with gas reaches temperatures up to 2000° C. at pressures of up to 50 bars and may comprise solid particles that are abrasive. To control the operation of the combustor the actual temperature Tgas (t) of the gas inside the combustor has to be monitored in real time.
The above mentioned circumstances make it difficult to measure the temperature by means of thermo-elements or optical instruments.
To overcome these difficulties it is known to determine the temperature of the gas based on its sonic speed, since the temperature is a function of the sonic speed.
From U.S. Pat. No. 4,317,366 a method and system for measuring temperatures in combustion chambers is known.
This method monitors the acoustic pressure spectrum inside a combustion chamber during operation and determines based on at least one maximum of the acoustic pressure spectrum (resonance frequency) the temperature of the gas inside the chamber.
From U.S. Pat. No. 6,979,118 B1 a method for estimating combustor flame temperature is known. This method comprises the determination of the resonance frequency of the first transverse acoustic mode of the combustor and determines the temperature of the gas based on this resonance frequency.
Since these methods don't comprise active acoustic excitation their precision and significance is strongly limited. If the acoustic power spectrum is measured only passively, the highest power level in the spectrum might be found at the first transversal resonance frequency at an axial location close to the burner mouth. At this location the radial temperature distribution is strongly inhomogeneous due to areas of unburned gas and thus the measured temperature might significantly deviate from the flame temperature.
Document GB 931233 describes a temperature measurement within a resonance cavity which might be exposed to hot combustion gases. The described temperature measurement is spatially averaged over the resonance cavity length and height. Consequently, dynamic temperature measurements or real time control of a combustor is not possible with such a method. The same disadvantage is valid for the method known from U.S. Pat. No. 4,164,867.